在Python中如何使用抽象基类实现dict? [英] How would I implement a dict with Abstract Base Classes in Python?
问题描述
dict
类,以便清楚,使用抽象基础课程? >>> D类(collections.MutableMapping):
... pass
...
>>> d = D()
追溯(最近的最后一次调用):
文件< stdin>,第1行,< module>
TypeError:无法使用抽象方法实例化抽象类D __delitem__,__getitem__,__iter__,__len__,__setitem__
一个好的答案将演示如何使这项工作,特别是没有子类化 dict
(一个我非常熟悉的概念)。
如何使用抽象基类实现dict?
一个好的答案将演示如何使这项工作,特别是
没有子类化dict。
这里是错误消息: TypeError:无法使用抽象方法__delitem__,__getitem__,__iter__,__len__,__setitem __
事实证明,必须实施它们来使用Abst基本类(ABC),
MutableMapping
。 实施
所以我实现了一个在大多数方面使用对象的属性引用dict进行映射的映射。 (代理与继承不同,所以我们只需要委托给实例 __ dict __
,我们可以使用任何其他的临时映射,但是你似乎没有在Python 2中这样做是有道理的,因为MutableMapping在Python 2中没有 __ slots __
,所以你正在创建一个在Python 3中,您可以通过设置 __ slots __
来避免使用错误。)
import collections
class D(collections.MutableMapping):
'''
映射工作像一个dict和一个可变对象,即
d = D(foo ='bar')
和
d.foo返回'bar'
'''
从类中创建实例所需的#``__init__``方法。
def __init __(self,* args,** kwargs):
'''使用对象dict'''
self .__ dict __。update(* args,** kwargs)
#接下来的五个方法是ABC的要求。
def __setitem __(self,key,value):
self .__ dict __ [key] = value
def __getitem __(self,key):
return self .__ dict __ [key]
def __delitem __(self,key):
del self .__ dict __ [key]
def __iter __(self):
return iter(self .__ dict__)
def __len __自我)
return len(self .__ dict__)
#最后两个方法不是必需的,但对于演示目的很有用:
def __str __(self):
' '返回简单的表示映射'''
return str(self .__ dict__)
def __repr __(self):
'''echoes class,id,& REPL'''
return'{},D({})'中的可重复表示格式(super(D,self).__ repr __(),
self .__ dict__)
演示
并演示用法:
>>> d = D((e,i)for i,e in enumerate('abc'))
>>>> D({'b':1,'c':2,'a':0})
>>>>> d.a
0
>>> d.get('b')
1
>>> d.setdefault('d',[])。append(3)
>>> d.foo ='bar'
>>> print(d)
{'b':1,'c':2,'a':0,'foo':'bar','d':[3]}
为了确保dict API,获得的经验教训是可以随时检查 collections.MutableMapping
>>> isinstance(d,collections.MutableMapping)
True
>>> isinstance(dict(),collections.MutableMapping)
True
而一个dict总是由于注册收集导入,将成为MutableMapping的一个实例,反之并不总是如此:
>> > isinstance(d,dict)
False
>>> isinstance(d,(dict,collections.MutableMapping))
True
执行此练习后,很明显,使用抽象基类只能为类的用户提供标准API的保证。在这种情况下,假设MutableMapping对象的用户将被保证为Python的标准API。
注意事项:
fromkeys
类构造方法未实现。
>> > dict.fromkeys('abc')
{'b':无,'c':无,'a':无}
>>> D.fromkeys('abc')
追溯(最近的最后一次调用):
文件< stdin>,第1行,< module>
AttributeError:类型对象'D'没有属性'fromkeys'
内置dict方法,如 get
或 setdefault
>>> d ['get'] ='baz'
>>> d.get('get')
追溯(最近的最后一次调用):
文件< stdin>,第1行,< module>
TypeError:'str'对象不可调用
重新取消屏蔽相当简单: / p>
>>> del d ['get']
>>> d.get('get','不在,但工作')
'不在,但工作'
但是我不会在生产中使用这个代码。
没有dict的演示,Python 3 :
>>> class MM(MutableMapping):
... __delitem__,__getitem__,__iter__,__len__,__setitem__ =(无,)* 5
... __slots__ =()
...
>>> MM().__ dict__
追溯(最近的最后一次调用):
文件< stdin>,第1行,< module>
AttributeError:'MM'对象没有属性'__dict__'
I attempted to implement a mapping in Python by using the abstract base class, MutableMapping, but I got an error on instantiation. How would I go about making a working version of this dictionary that would emulate the builtin dict
class, in as many ways as possible, to be clear, with Abstract Base Classes?
>>> class D(collections.MutableMapping):
... pass
...
>>> d = D()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class D with abstract methods __delitem__, __getitem__, __iter__, __len__, __setitem__
A good answer will demonstrate how to make this work, specifically without subclassing dict
(a concept that I am quite familiar with).
How would I implement a dict with Abstract Base Classes?
A good answer will demonstrate how to make this work, specifically without subclassing dict.
Here's the error message: TypeError: Can't instantiate abstract class D with abstract methods __delitem__, __getitem__, __iter__, __len__, __setitem__
It turns out that one must implement them to use the Abstract Base Class (ABC), MutableMapping
.
Implementation
So I implement a mapping that works like a dict in most respects that uses the object's attribute reference dict for the mapping. (Delegation is not the same as inheritance, so we'll just delegate to the instance __dict__
, we could use any other ad-hoc mapping, but you don't seem to care about that part of the implementation. It makes sense to do it this way in Python 2, because MutableMapping doesn't have __slots__
in Python 2, so you're creating a __dict__
either way. In Python 3, you could avoid dicts altogether by setting __slots__
.)
import collections
class D(collections.MutableMapping):
'''
Mapping that works like both a dict and a mutable object, i.e.
d = D(foo='bar')
and
d.foo returns 'bar'
'''
# ``__init__`` method required to create instance from class.
def __init__(self, *args, **kwargs):
'''Use the object dict'''
self.__dict__.update(*args, **kwargs)
# The next five methods are requirements of the ABC.
def __setitem__(self, key, value):
self.__dict__[key] = value
def __getitem__(self, key):
return self.__dict__[key]
def __delitem__(self, key):
del self.__dict__[key]
def __iter__(self):
return iter(self.__dict__)
def __len__(self):
return len(self.__dict__)
# The final two methods aren't required, but nice for demo purposes:
def __str__(self):
'''returns simple dict representation of the mapping'''
return str(self.__dict__)
def __repr__(self):
'''echoes class, id, & reproducible representation in the REPL'''
return '{}, D({})'.format(super(D, self).__repr__(),
self.__dict__)
Demonstration
And to demonstrate the usage:
>>> d = D((e, i) for i, e in enumerate('abc'))
>>> d
<__main__.D object at 0x7f75eb242e50>, D({'b': 1, 'c': 2, 'a': 0})
>>> d.a
0
>>> d.get('b')
1
>>> d.setdefault('d', []).append(3)
>>> d.foo = 'bar'
>>> print(d)
{'b': 1, 'c': 2, 'a': 0, 'foo': 'bar', 'd': [3]}
And for ensuring the dict API, lesson learned is that you can always check for collections.MutableMapping
:
>>> isinstance(d, collections.MutableMapping)
True
>>> isinstance(dict(), collections.MutableMapping)
True
And while a dict is always going to be an instance of a MutableMapping due to registration on collections import, the reverse is not always true:
>>> isinstance(d, dict)
False
>>> isinstance(d, (dict, collections.MutableMapping))
True
After performing this exercise, it is clear to me that using Abstract Base Classes provides only the guarantee of a standard API for users of the class. In this case, users assuming a MutableMapping object will be guaranteed the standard API for Python.
Caveats:
The fromkeys
class constructor method is not implemented.
>>> dict.fromkeys('abc')
{'b': None, 'c': None, 'a': None}
>>> D.fromkeys('abc')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: type object 'D' has no attribute 'fromkeys'
One could mask the builtin dict methods like get
or setdefault
>>> d['get'] = 'baz'
>>> d.get('get')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable
It's fairly simple to unmask again:
>>> del d['get']
>>> d.get('get', 'Not there, but working')
'Not there, but working'
But I wouldn't use this code in production.
Demonstration without a dict, Python 3:
>>> class MM(MutableMapping):
... __delitem__, __getitem__, __iter__, __len__, __setitem__ = (None,) *5
... __slots__ = ()
...
>>> MM().__dict__
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'MM' object has no attribute '__dict__'
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